[NumLock]

Код:

procedure TForm1.Button1Click(Sender: TObject);
var
  FSaveNotifyEvent: TNotifyEvent;
begin
  FSaveNotifyEvent:=CheckBox1.OnClick;
  CheckBox1.OnClick:=nil;
  CheckBox1.Checked:=not CheckBox1.Checked;
  CheckBox1.OnClick:=FSaveNotifyEvent;
end;